1. Field of the Invention
The present invention relates to a hot working die steel which is used in the state of relatively high temperatures (suitably 300° C. or more) and a member for high temperature use such as a structural member for a casting machine, a structural member for an injection molding machine, and a member for a hot forging machine which are made of the hot working die steel.
The present application is based on Japanese Patent Application No. 2001-133945, the entire contents of which are incorporated herein by reference.
2. Description of Related Art
As a structural member for a casting machine which is exposed to high temperatures when aluminum, magnesium, or an alloy having them as principal components is fabricated by casting, a hot working die steel such as 5% Cr-based JIS (Japanese Industrial Standards)-SKD 61 steel has been conventionally adopted. In addition, the JIS-SKD 61 steel has similarly been adopted as a structural member for an injection molding machine for such a light metal or a low melting metal.
In the cases where the JIS-SKD 61 steel is used for such applications, its life expires due to various factors, and as one factor it is possible to cite the shortage of creep rupture ductility and an increase in the creep strain occurring when the JIS-SKD 61 steel is used for extended periods of time in an environment in which stresses are applied at elevate temperatures. This is because although, in the JIS-SKD 61 steel, an attempt is made to reinforce it by allowing carbides to precipitate in the martensite in the form of very fine particles by tempering, if it is used for extended periods of time at evaluated temperature, the recovery of the dislocation and the coagulation and coarsening of carbides occur, so that the initial material property cannot be maintained, and the JIS-SKD 61 steel gradually softens. Furthermore, in such as an injection machine which is heated to high temperatures, there emerges the risk that the JIS-SKD 61 steel is subjected to abrasive scoring and is liable to be broken in the process in which a shearing force is imparted to a solid alloy and the solid alloy is melted.
Meanwhile, a Ni-base superalloy such as Inconel 718 (trade name, hereinafter the same) is known as materials excelling in the strength at elevated temperatures. However, this material has a problem in that its corrosion is noticeable due to the molten aluminum, magnesium, or alloy having them as principal components. Furthermore, when the structural member is heated by a heater or the like to melt aluminum, magnesium, or an alloy having them as principal components, the Ni-base superalloy such as Inconel 718 is poor in thermal conductivity and low in ductility and toughness at high temperatures. Hence, there have been problems in that thermal stresses attributable to a temperature difference between inner and outer surfaces of the member occur, and that the material deteriorates and the reliability as the structural member declines. In addition, although it is known that Stellite (trade name, hereinafter the same), which is generally used as a tool material and a valve material, and other cobalt-base alloys, are materials excellent in the strength at elevated temperatures and that their quantity of molten corrosion is small, these metals have a problem in that they are difficult to use as structural members since their toughness is slightly inferior and they are expensive.